Is cartridge Demagnetizing Necessary?

NO never.

Just don’t demagnetize a moving magnet cartridge, or you will have no magnet and no cartridge. The rest of it is up to you. I would never use a demagnetizer per se. I occasionally use the tracks on the Cardas test record which are specifically for demagnetizing.

Ironically, there is a business centered around routing the audio signal thru cables surrounded by powerful magnets. Go figure.

lewm

Just don’t demagnetize a moving magnet cartridge, or you will have no magnet and no cartridge.

This is completely mistaken.
MM cartridges can be safely demagnetized - see the instruction manual for details. The Sumiko Fluxbuster manual specifically cautions against demagnetizing MM cartridges until the stylus assembly is removed first.

OK. Everyone should read the Sumiko Fluxbuster manual, because after all they have no agenda in the debate.  Bust all the flux you want.

1+++lewm. The entire concept of "demagnetizing" a cartridge is beyond silly. The only thing you can hope to do with a real demagnetizer (like we use to use on tape heads) is ruin the cartridge. This is a typical example of audiophile mythology because it seems many audiophiles are not able to determine what is real and what is illusion.